Refrigeration cycle for liquid oxygen densification

ABSTRACT

Closed-loop refrigeration cycles for liquid oxygen densification are disclosed. The disclosed refrigeration cycles may be turbine-based refrigeration cycles or a Joule-Thompson (JT) expansion valve based refrigeration cycles and include a refrigerant or working fluid comprising a mixture of neon or helium together with nitrogen and/or oxygen.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. Provisional PatentApplication Ser. No. 62/446,586 filed on Jan. 16, 2017, the disclosureof which is incorporated by reference herein.

TECHNICAL FIELD

The present invention is related to cryogenic refrigeration cycles forpropellant densification, and more particularly to systems and methodsfor densifying liquid oxygen using a closed-loop refrigeration circuitthat includes a refrigerant or working fluid comprising a mixture ofneon or helium together with nitrogen and/or oxygen.

BACKGROUND

Cryogenic refrigeration systems have been used for decades in manyrocket or space applications for purposes such as liquefaction,propellant subcooling (i.e. thermal control), propellant densificationand to prevent propellant boil off (i.e. conserve propellant). Forpropellant densification, the removal of sensible heat from a liquidpropellant, such as liquid oxygen, increases the density of the liquidpropellant, which is solely dependent on temperature since liquids aregenerally considered incompressible. Challenges facing the spaceindustry related to propellant densification include reducing theoperational and capital cost of propellant densification as well asreducing the time it takes to achieve the desired temperatures.

As a result of these challenges, there is a continuing need to developimproved refrigeration cycles for densification of propellants, such asliquid oxygen.

SUMMARY OF THE INVENTION

The present invention may be characterized as a closed-loopJoule-Thompson (JT) expansion valve based refrigeration system forliquid oxygen densification comprising: (i) a compressor having an inletand an outlet and configured to receive a working fluid at the inlet andcompress the working fluid at a pressure ratio between the inlet andoutlet of between 10.0 and 25.0; (ii) a heat exchanger in fluidcommunication with the outlet of the compressor configured to receivethe compressed working fluid and cool the compressed working fluid viaindirect heat exchange with a gaseous nitrogen stream or a liquidnitrogen stream or both to produce a cold, compressed working fluid;(iii) a JT expansion valve in fluid communication with the heatexchanger and configured to expand the cold, compressed working fluidand produce a refrigeration stream of expanded working fluid; (iv) anoxygen chiller in fluid communication with the JT expansion valve, theoxygen chiller configured to receive a stream of liquid oxygen and therefrigeration stream from the JT expansion valve, and subcool the streamof liquid oxygen to a temperature less than 66.5 K, preferably less than62.0 K, more preferably less than or equal to 60.9 K via indirect heatexchange with the refrigeration stream of expanded working fluid toproduce a densified liquid oxygen stream; and (v) a recirculatingconduit connecting the oxygen chiller with the heat exchanger and theinlet of the compressor, the recirculating conduit configured torecirculate the warmed refrigeration stream first to the heat exchangerwhere it further cools the compressed working fluid and then to theinlet of the compressor where the warmed refrigeration stream iscompressed as the working fluid.

The present invention may also be characterized as a method ofdensifying a liquid oxygen stream in a closed-loop JT expansion valvebased refrigeration cycle, the method comprising the steps of: (a)compressing a working fluid having between 70 mol % and 80 mol % neon orneon and helium and between 20 mol % and 30 mol % nitrogen and/or oxygenin a multistage compressor from a pressure just above ambient pressureto a pressure between about 150 psia and 380 psia; (b) cooling thecompressed working fluid via indirect heat exchange with a cold gaseousnitrogen stream or a liquid nitrogen stream or both to produce a cold,compressed working fluid; (c) expanding the cold, compressed workingfluid in a JT expansion valve to produce a refrigeration stream ofexpanded working fluid at a temperature less than 66.5 K, preferablyless than 62.0 K, more preferably less than or equal to 60.9 K; (d)subcooling a stream of liquid oxygen via indirect heat exchange with theexpanded working fluid to produce a densified liquid oxygen stream; and(e) recirculating the warmed refrigeration stream to the multi-stagecompressor, wherein the warmed refrigeration stream is compressed as theworking fluid to form a closed-loop refrigeration cycle.

Alternatively, the present invention may be characterized as a turbinebased closed-loop refrigeration system for liquid oxygen densificationcomprising: (i) a compressor configured to compress a working fluidhaving between 85 mol % and 95 mol % neon and/or helium and between 5mol % and 15 mol % nitrogen and/or oxygen from a pressure just aboveambient pressure to a pressure between about 120 psia and 155 psia; (ii)a heat exchanger in fluid communication with the outlet of thecompressor configured to receive the compressed working fluid and coolthe compressed working fluid via indirect heat exchange with a gaseousnitrogen stream or a liquid nitrogen stream or both to produce a cold,compressed working fluid; (iii) a turbine in fluid communication withthe heat exchanger and configured to expand the cold, compressed workingfluid and produce a refrigeration stream of expanded working fluid; (iv)an oxygen cooler in fluid communication with the turbine, the oxygencooler configured to receive a stream of liquid oxygen and therefrigeration stream from the turbine, and subcool the stream of liquidoxygen to a temperature less than 66.5 K, preferably less than 62.0 K,more preferably less than or equal to 60.9 K via indirect heat exchangewith the refrigeration stream of expanded working fluid to produce adensified liquid oxygen stream; (v) a recirculating conduit configuredto recirculate the warmed refrigeration stream to an inlet of thecompressor where the warmed refrigeration stream is compressed as theworking fluid.

Finally, the present invention may be characterized as a method ofdensifying a liquid oxygen stream in a closed-loop, turbine basedrefrigeration cycle, the method comprising the steps of: (a) compressinga working fluid having between about 85 mol % and about 95 mol % neonand/or helium and between about 5 mol % and about 15 mol % nitrogenand/or oxygen in a compressor from a pressure just above ambientpressure to a pressure between about 120 psia and 155 psia; (b) coolingthe compressed working fluid via indirect heat exchange with a gaseousnitrogen stream or a liquid nitrogen stream or both to produce a cold,compressed working fluid; (c) expanding the cold, compressed workingfluid in a turbine to produce a refrigeration stream of expanded workingfluid at a temperature less than 66.5 K, preferably less than 62.0 K,more preferably less than or equal to 60.9 K; (d) subcooling a stream ofliquid oxygen via indirect heat exchange with the expanded working fluidto produce a densified liquid oxygen stream; and (e) recirculating thewarmed refrigeration stream to the compressor, wherein the warmedrefrigeration stream is compressed as the working fluid to form aclosed-loop refrigeration cycle.

In some embodiments of the present systems and methods the compressor isa cold compressor configured to receive the working fluid at atemperature of between about 65 K and 80 K and a pressure just aboveambient pressure at the inlet. Other embodiments contemplate a warmercompressor configured to receive the working fluid at a temperature ofbetween about 100 K and 310 K and a pressure just above ambient pressureat the inlet. Where warmer compression is contemplated, the workingfluid may be required to be further warmed in the heat exchanger or arecuperator upon exiting the oxygen chiller/cooler prior to it beingdirected to the inlet of the compressor. Use of the recuperator eitheras a stand-alone heat exchanger or as an integrated part of the mainheat exchanger is also preferred in the JT based closed looprefrigeration cycle.

The working fluid is preferably selected from the following mixtures:(i) a mixture of neon with nitrogen; (ii) a mixture of neon with oxygen;(iii) a mixture of neon with nitrogen and oxygen; (iv) a mixture ofhelium with nitrogen; (v) a mixture of helium with oxygen; (vi) amixture of helium with nitrogen and oxygen; or (vii) a mixture of neonand helium with nitrogen and/or oxygen.

In the JT based refrigeration cycles, the JT expansion valve ispreferably configured such that the expanded working fluid exiting theJT expansion valve is between about 15 mol % to about 30 mol % liquid,whereas in the turbine based refrigeration cycles, the expanded workingfluid exiting the turbine is preferably between about 5 mol % to about10 mol % liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims specifically pointing outthe subject matter that Applicant regards as his invention, it isbelieved that the invention will be better understood when taken inconnection with the accompanying drawings in which:

FIG. 1 is a schematic flow sheet of a turbine based refrigeration cyclefor liquid oxygen densification with cold compression in accordance withone embodiment of the invention;

FIG. 2 is a schematic flow sheet of a turbine based refrigeration cyclefor liquid oxygen densification with warm compression in accordance withanother embodiment of the invention;

FIG. 3 is a schematic flow sheet of a Joule-Thompson (JT) expansionvalve based refrigeration cycle for liquid oxygen densification withcold compression in accordance with yet another embodiment of theinvention; and

FIG. 4 is a schematic flow sheet of a Joule-Thompson (JT) expansionvalve based refrigeration cycle for liquid oxygen densification withwarm compression in accordance with a further embodiment of theinvention.

DETAILED DESCRIPTION

The present system and method for densifying liquid oxygen employs a lowtemperature, closed-loop refrigeration circuit that includes arefrigerant or working fluid comprising a mixture of neon or heliumtogether with nitrogen and/or oxygen. This closed-loop refrigerationcircuit can be configured as a supplemental refrigeration system coupledto an existing densification system or may be a stand-alonerefrigeration system. In either arrangement, the closed-looprefrigeration system is sized to reduce the time it takes for liquidoxygen densification and reach the target temperature.

As described in more detail below, the low temperature, closed-looprefrigeration system may include a turbine based refrigeration circuitwith cold compression or warm compression. Alternatively, the lowtemperature, closed-loop refrigeration system may include a JT expansionvalve based refrigeration circuit with cold compression or warmcompression.

The closed-loop turbine based refrigeration cycles are more powerefficient but have higher capital costs than the closed-loop JTexpansion valve based refrigeration cycles. In addition, the turbinebased refrigeration cycles present a technical challenge with respect toliquid forming within the turbine or upstream of the turbine which canbe addressed by the selection of the proper refrigerant or working fluidmixture. The JT expansion valve based refrigeration cycles on the otherhand are lower in capital costs but present higher operating costs (i.e.power costs) and higher liquid nitrogen consumption. Advantageously, theJT expansion valve refrigeration cycles are also more tolerable ofdifferent refrigerant or working fluid mixtures.

The refrigerant or working fluid in the low temperature, closed looprefrigeration cycles described herein are preferably: (i) a mixture ofneon with nitrogen; (ii) a mixture of neon with oxygen; (iii) a mixtureof neon with nitrogen and oxygen; (iv) a mixture of helium withnitrogen; (v) a mixture of helium with oxygen; (vi) a mixture of heliumwith nitrogen and oxygen; or (vii) a mixture of neon and helium withnitrogen and/or oxygen.

For the working fluids described above, use of nitrogen would be muchpreferred compared to the use of oxygen in combination with neon orhelium because it becomes two phase at a lower temperature (boilingpoint of nitrogen is 77.4 K and boiling point of oxygen is 90.2 K). Inthe turbine based refrigeration cycles described below with reference toFIGS. 1 and 2, this means that the desired cold end temperature can beachieved without liquid formation upstream of the turbine or within theturbine nozzles, before the impeller. When significant amounts of oxygenare included in the mixture it can become difficult to avoid liquidformation in these locations. As a result, a higher pressure ratioturbine may be needed, or a turbine technology that can accept liquidformation early in its expansion process.

The problem with a working fluid of a mixture of only nitrogen with neonor helium is the likelihood of the working fluid forming a solid phaseat cold end temperatures. To mitigate the problem of solid phaseformation in a closed loop refrigeration cycle, some amount of oxygen inthe working fluid may be preferred or even required.

Turbine Based Refrigeration Cycles

FIG. 1 shows a turbine based refrigeration cycle 10 configured todensify a stream of liquid oxygen 32. The working fluid (i.e. streams14,16,18,26) preferably contains between 5 mol % and 15 mol % nitrogenand/or oxygen in either neon or helium. This working fluid circulatesthrough the closed loop refrigeration circuit 12. The cold compressor 15raises the pressure of the working fluid, which is preferably just aboveatmospheric pressure, by a pressure ratio of about 10.0. For example, aworking fluid stream 14 fed to the cold compressor 15 at a pressure ofabout 15.2 psia would be compressed and form a compressed working fluidstream 16 at a pressure of about 152 psia (e.g. 150 psia to 155 psia).Operating the cold compressor 15 at a compressor pressure ratio of about10.0 allows the turbine pressure ratio to approach 10.0, which is aboutthe maximum pressure ratio that current designs of radial inflowturbines can operate efficiently. In the illustrated embodiments, thecompressor 25 is shown as a motor 11 driven compressor 15 while theturbine is operatively coupled to a generator 27.

After cold compression, the compressed working fluid stream 16 iswarmer, but still at a temperature that is preferably below ambienttemperature. This compressed working fluid stream 16 is then cooled tonear 80 K in the main cooler heat exchanger 20 by indirect heat exchangewith a stream of liquid nitrogen 21 in addition to any cold gas nitrogenstream 23 that may be available at the facility/site. The cold,compressed working fluid 18 is then expanded across the turbine 25 toproduce a cold working fluid exhaust stream 26 at a pressure just aboveambient pressure. The exhaust stream 26 from the turbine 25 is alsopartially condensed to between about 5 mol % to about 10 mol % liquid,depending on the amount of nitrogen and/or oxygen in the working fluidand the operating conditions of the turbine 25. The cold turbine exhauststream 26 is then passed through an oxygen chiller heat exchanger 30,where the influent liquid oxygen stream 32 is cooled and densified byindirect heat exchange with the cold working fluid exhaust stream 26. Inthis manner, the densified liquid oxygen stream 34 is chilled to atemperature less than 66.5 K, preferably less than 62.0 K, morepreferably less than or equal to 60.9 K. The circulating working fluid14 is warmed in the oxygen chiller to a slightly superheated state,preferably to a temperature between about 60 K and about 75 K and at apressure just above atmospheric pressure

Less nitrogen and/or oxygen and more neon or helium in the recirculatingworking fluid within the closed loop refrigeration circuit 12 enablescooling of the liquid oxygen to lower temperatures. The benefit of theheavier components (i.e. nitrogen and/or oxygen) is to provide latentheat in the oxygen chiller heat exchanger 30. In other words, lessnitrogen and/or oxygen concentrations in the recirculating working fluidwould require a greater recirculating flow and therefore greater powerconsumption to achieve the desired densification.

The viability of the turbine based refrigeration cycle 10 for liquidoxygen densification with cold compression as shown in the embodiment ofFIG. 1, very much depends on the flow and temperature of the availablecold gas nitrogen. The primary benefit of the turbine basedrefrigeration cycle 10 for liquid oxygen densification with coldcompression is reduced power consumption compared to a similararrangement with warm compression. However, if too much liquid nitrogenis required to assist in the indirect heat transfer in the main coolerheat exchanger 20, it may not be economical to use the cold compressionarrangement to achieve the desired liquid oxygen densification.Currently, most liquid oxygen densification systems use sub-atmosphericliquid nitrogen to cool the oxygen. Cold gas nitrogen may be availableas an otherwise vented stream from these systems when this closed-loopsystem is supplied for supplemental cooling.

FIG. 2 shows an alternative embodiment that utilizes warmer compressionin lieu of the cold compression arrangement shown in FIG. 1. While manyof the features and descriptions of the embodiment of FIG. 1 are similarto the embodiment of FIG. 2, for the sake of brevity such similardescriptions will not be repeated. Rather, the following discussion willfocus on the differences between the embodiments of FIG. 1 and FIG. 2.

In the embodiment shown in FIG. 2, the warmed, working fluid 14 exitingthe oxygen chiller heat exchanger 30 is further warmed via indirect heatexchange in the main cooler heat exchanger 20 thereby providing thenecessary cooling of the compressed working fluid 16 prior to its feedto the turbine 25. Preferably, the further warmed refrigerant stream 13remains below ambient temperature exiting the main cooler heat exchanger20. However, this will only be possible if cold nitrogen gas 23 isavailable. If so, it will warm due to the heat of compression so that itrequires cooling in an aftercooler 17 upon exit of the compressor 15,and/or intercooling between some of the stages of compression in amulti-stage compressor system.

In the warmer compression embodiment of FIG. 2, a source of liquidnitrogen for supplemental refrigeration in the main cooler heatexchanger is not needed as in the cold compression embodiment of FIG. 1.The turbine generates enough refrigeration to cool the oxygen andsustain the system. The embodiment of FIG. 2 also shows that the furtherwarmed working fluid stream 13 is withdrawn from the main cooler heatexchanger 20 at an intermediate location 29 before reaching the warm endof the main cooler heat exchanger, which allows the temperature to theworking fluid fed to the compressor 15 or first stage of the compressorsystem to be lower than it would otherwise be.

Joule-Thompson (JT) Expansion Valve Based Refrigeration Cycles

The illustrated embodiments in FIG. 3 and FIG. 4 represent the JTexpansion valve based refrigeration cycles. In the embodiment of FIG. 3a cold compression JT expansion valve based refrigeration system 50 isshown that comprises a motor 53 driven cold compressor 55 configured tocompress the refrigerant or working fluid; a main cooler heat exchanger60 configured to cool the compressed working fluid 56; a recuperator 80configured to provide further cooling of the cold, compressed workingfluid 58; a JT throttling valve 65 used to expand the further cooled,compressed working fluid 59 and produce the refrigeration; and an oxygenchiller heat exchanger 70 to indirectly transfer the refrigeration fromthe expanded working fluid 66 to the liquid oxygen stream 72 for coolingand densification thus producing a densified liquid oxygen stream 74.

The cold compressor 55 is preferably a multi-stage compressor systemthat receives a stream of refrigerant or working fluid 53 at asub-ambient temperature (e.g. between about 65 K and 80 K) and apressure just above atmospheric pressure and raises the pressure of theworking fluid by a ratio of between about 10.0 to 25.0. For example, ifthe working fluid 53 fed to the cold compressor 55 is at a pressure ofabout 15.2 psia, the cold compressor would compress the working fluid toa pressure between about 150.0 psia and 380.0 psia. After compression,the compressed working fluid stream 56 is warmer, but still preferablyat a temperature below ambient temperature. This compressed workingfluid stream 56 is then cooled to a temperature near 80 K in the maincooler heat exchanger 60 via indirect heat exchange with a stream ofliquid nitrogen 61 and/or a stream of cold gaseous nitrogen 63.

Further cooling of the cold, compressed working fluid 58 to temperaturesbelow that of the liquid nitrogen together with partial condensation isaccomplished in the recuperator 80. As described in more detail below,the cooling medium in the recuperator 80 is the expanded working fluid54. While the recuperator 80 and main cooler heat exchanger 60 are shownas separate heat exchangers, it is possible and fully contemplated thatsuch cooling functions may be combined within a single heat exchanger.

The further cooled working fluid 59 is directed to the JT expansionvalve 65 where the working fluid is expanded to produce therefrigeration. The expanded working fluid 66 exiting the JT expansionvalve 65 is preferably between about 15 mol % to about 30 mol % liquid,depending on the amount of nitrogen and/or oxygen in the working fluid.Refrigeration production, in reference a JT expansion process does notextract energy from the working fluid. Refrigeration production heresimply means that a colder temperature results from the JT expansionprocess. The cold, two-phase, expanded working fluid 66 is directed tothe oxygen chiller heat exchanger 70 where it provides cooling (i.e.densification) of the liquid oxygen stream 72. As shown and described,the JT expansion valve based refrigeration cycle 50 is capable ofreaching the target liquid oxygen temperature of less than 66.5 K,preferably less than 62.0 K, more preferably less than or equal to 60.9K, and even somewhat below the target liquid oxygen temperature.

Upon exiting the oxygen chiller heat exchanger 70, the warmed workingfluid 54 is directed to the recuperator 80 to provide the furthercooling of the compressed working fluid 58. Upon exiting the recuperator80, the further warmed working fluid 53 in this closed looprefrigeration system 52 is then fed back to cold compressor 55,preferably at a temperature of between about 65 K and 80 K.

The working fluid for the JT expansion valve based refrigeration cyclesis preferably a mixture of neon (i.e. light component) and nitrogenand/or oxygen (i.e. heavy component). Helium may be combined with neonas another light component. The light components or neon enables coolingof the working fluid to the very cold temperatures needed. The heavycomponents provide latent heat in the oxygen chiller heat exchanger. Inthe oxygen chiller heat exchanger, liquid oxygen is cooled and densifiedvia indirect heat exchange with the cold, expanded working fluid. Higherconcentrations of the heavy components (i.e. nitrogen and/or oxygen) inthe working fluid reduce the flow needed to densify the liquid oxygenand also reduces the compression power needed to compress the workingfluid in the cold compressor. However, higher concentrations of theheavy components (i.e. nitrogen and/or oxygen) also limit the cold endtemperatures attainable and thus the extent of liquid oxygendensification that can be achieved.

The optimal circulating refrigerant or working fluid contains betweenapproximately 20 mol % to 30 mol % of the heavy component (i.e.nitrogen, oxygen or a combination of nitrogen and oxygen), with theremainder being the light component. Helium is much less effective thanneon in the JT throttling valve refrigeration cycle because heliumprovides much less JT refrigeration at these temperatures. It can onlybe used in combination with neon, not as the sole light component.

The economic viability of the cold compression configuration shown inFIG. 3 depends on the flow and temperature of the available cold gasnitrogen. The primary benefit of cold compression is the reduced powerconsumption compared to a warm compression arrangement, shown in FIG. 4.Supplemental liquid nitrogen is preferably used in addition to the coldgaseous nitrogen to cool the compressed working fluid. However, if toomuch liquid nitrogen is required to assist in the main cooler heatexchanger in reaching the desired temperatures, it will not beeconomical to use the cold compression arrangement.

FIG. 4 shows an alternative embodiment of the closed-loop JT expansionvalve based refrigeration system 50 that utilizes warmer compression inlieu of the cold compression arrangement shown in FIG. 3. While many ofthe features and descriptions of the embodiment shown in FIG. 3 aresimilar to the embodiment shown in FIG. 4, for the sake of brevity suchdescriptions will not be repeated. Rather, the following discussion willfocus on the differences between the embodiments of FIG. 3 and FIG. 4.

In the embodiment of FIG. 4 a warmer compression JT expansion valvebased refrigeration system 50 is shown that comprises a motor 51 drivencompressor 55 configured to compress the refrigerant or working fluid53; a combined recuperator and main cooler heat exchanger 60,80configured to cool the compressed working fluid 56; a JT throttlingvalve 65 used to expand the working fluid and produce the refrigeration;and an oxygen chiller heat exchanger 70 to indirectly transfer therefrigeration from the expanded working fluid 66 to the liquid oxygenstream 72 for cooling and densification and produce a densified liquidoxygen stream 74. The compressor 55 is preferably a multi-stagecompressor system that receives a stream of refrigerant or working fluid53 at a pressure just above atmospheric pressure and a sub-ambienttemperature, although warmer than the corresponding stream in theearlier described cold compressor embodiments. Depending on theavailability of cold gas nitrogen and cost of liquid nitrogen, theworking fluid may alternatively be supplied at ambient temperature.Similar to the cold compression embodiment, the pressure ratio acrossthe compressor 55 is also between about 10.0 and 25.0.

The compressed working fluid stream 56 is then cooled to a temperatureagain near 80 K in the combined recuperator and main cooler heatexchanger 60,80 via indirect heat exchange with a stream of liquidnitrogen 61 and/or a stream of cold gaseous nitrogen 63 and thereturning warmed working fluid 54. Liquid nitrogen will still berequired to achieve the desired level of cooling of the compressedworking fluid 56 in the combined recuperator and main cooler heatexchanger 60,80 although the demand for liquid nitrogen will be muchlower than in the cold compression embodiment of FIG. 3.

In the embodiment shown in FIG. 4, the warmed, working fluid exiting theoxygen chiller heat exchanger 70 is further warmed via indirect heatexchange in the combined recuperator and main cooler heat exchanger60,80 thereby providing the necessary cooling of the compressed workingfluid 59 prior to expansion in the JT throttling valve 65. The furtherwarmed, expanded working fluid 53 may remain below ambient temperatureexiting the combined recuperator and main cooler heat exchanger 60,80.If so, it will warm due to the heat of compression so that it requirescooling in an aftercooler 57 upon exit of the motor 51 driven compressor55, and/or intercooling between some of the stages of compression in amulti-stage compressor system.

The embodiment of FIG. 4 also shows that the further warmed workingfluid stream is withdrawn from the combined recuperator and main coolerheat exchanger 60,80 at an intermediate location 69 before reaching thewarm end of the heat exchanger, which allows the temperature to theworking fluid fed to the compressor 55 or first stage of the compressorsystem to be lower than it would otherwise be. Alternatively, theworking fluid fed to the first stage of the compressor system could bewithdrawn at the warm end (approximately ambient temperature). Due tothe warmer temperature of the working fluid at the inlet of thecompressor, the power consumption of this version is significantlyhigher than that of the cold compression refrigeration cycle discussedabove with reference to FIG. 3.

While the present invention has been described with reference to apreferred embodiment or embodiments, it is understood that numerousadditions, changes and omissions can be made without departing from thespirit and scope of the present invention as set forth in the appendedclaims.

For example, operation of the closed-loop refrigeration cycles atsub-atmospheric low end pressures instead of pressures just aboveambient pressures would be helpful from a thermodynamic point of view,especially when oxygen is contained in the refrigerant or working fluidmixtures. Combining the features of sub-atmospheric low end pressureswith higher turbine pressure ratios, would enable the refrigerationsystem to reach the subcooling temperature without the higher saturationtemperature that would otherwise be required at the high pressure ratio.

Another means of achieving the desired low end temperature would havethe cold, compressed feed stream to the turbine being nearly at asaturated vapor point, such that liquid is formed early in the turbineexpansion process, or perhaps even with a small amount of liquid alreadypresent in the cold, compressed feed stream to the turbine. Employingthis feature or variation to the cold, compressed feed stream to theturbine may however require advanced turbine technologies.

A further variation is the inclusion of a turbine in addition to the JTexpansion valve. In this contemplated arrangement, a portion of theworking fluid stream could be withdrawn from the main cooler heatexchanger at an intermediate temperature and directed to a turbine whereit is exhausted at or below the liquid nitrogen temperature. In thisway, the liquid nitrogen usage could be reduced or eliminated.

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 12. A closed-loop refrigeration system for liquid oxygendensification comprising: a compressor configured to compress a workingfluid having between 85 mol % and 95 mol % neon and/or helium andbetween 5 mol % and 15 mol % nitrogen and/or oxygen from a pressureabove ambient pressure to a pressure between 120 psia and 155 psia; aheat exchanger in fluid communication with the outlet of the compressorconfigured to receive the compressed working fluid and cool thecompressed working fluid via indirect heat exchange with a gaseousnitrogen stream and a liquid nitrogen stream to produce a cold,compressed working fluid; a turbine in fluid communication with the heatexchanger and configured to expand the cold, compressed working fluidand produce a refrigeration stream of expanded working fluid; an oxygencooler in fluid communication with the turbine, the oxygen coolerconfigured to receive a stream of liquid oxygen and the refrigerationstream from the turbine, and subcool the stream of liquid oxygen to atemperature less than 66.5 K via indirect heat exchange with therefrigeration stream of expanded working fluid to produce a densifiedliquid oxygen stream; and a recirculating conduit configured torecirculate the warmed refrigeration stream to an inlet of thecompressor where the warmed refrigeration stream is compressed as theworking fluid.
 13. The closed-loop refrigeration system of claim 12,wherein the working fluid at the inlet of the compressor is at atemperature of between 65 K and 80 K.
 14. The closed-loop refrigerationsystem of claim 12, wherein the heat exchanger is further configured tofurther warm the warmed refrigeration stream via indirect heat exchangewith the stream of cold, compressed working fluid and wherein theworking fluid at the inlet of the compressor is at a temperature ofbetween 100 K and 310 K.
 15. The closed-loop refrigeration system ofclaim 12, wherein the working fluid is selected from the groupessentially consisting of: (i) a mixture of neon with nitrogen; (ii) amixture of neon with oxygen; (iii) a mixture of neon with nitrogen andoxygen; (iv) a mixture of helium with nitrogen; (v) a mixture of heliumwith oxygen; (vi) a mixture of helium with nitrogen and oxygen; or (vii)a mixture of neon and helium with nitrogen and/or oxygen.
 16. Theclosed-loop refrigeration system of claim 12, wherein the turbine isfurther configured such that the expanded working fluid exiting theturbine is between 5 mol % and 10 mol % liquid.
 17. A method ofdensifying a liquid oxygen stream in a closed-loop refrigeration cycle,the method comprising the steps of: compressing a working fluid havingbetween about 85 mol % and about 95 mol % neon and/or helium and between5 mol % and 15 mol % nitrogen and/or oxygen in a compressor from apressure above ambient pressure to a pressure between 120 psia and 155psia; cooling the compressed working fluid via indirect heat exchangewith a gaseous nitrogen stream or a liquid nitrogen stream or both toproduce a cold, compressed working fluid; expanding the cold, compressedworking fluid in a turbine to produce a refrigeration stream of expandedworking fluid at a temperature of less than 66.5 K; subcooling a streamof liquid oxygen via indirect heat exchange with the refrigerationstream of expanded working fluid to produce a densified liquid oxygenstream; and recirculating the warmed refrigeration stream to thecompressor; wherein the warmed refrigeration stream is compressed as theworking fluid to form a closed-loop refrigeration cycle.